Aqueous phase trapping (APT) as a kind of mechanical formation damage can occur in porous medium when water based well bore fluids come into contact with a formation which exhibits sub-irreducible initial water saturation. Such type of formation damage commonly occurs during drilling using water based fluids in tight gas reservoirs and in depleted conditions in rich gas retrograde condensate reservoirs and is a major mechanism of damage in these reservoirs.

Evaluating reservoir's potential for APT is of great importance where the reservoir is susceptible to APT, since well productivity can be improved through proper evaluating and consequent attempts to reduce associated formation damage. This can be done using APT laboratory tests on reservoir rock samples. But, due to some restrictions, an alternative approach is followed which is use of predictive formulas. During the last fifteen years, number of formulas has been proposed to evaluate the reservoir's potential for APT.

In this paper a full procedure for evaluating and minimizing formation damage through APT is described. Different formulas used for evaluating the sensitivity of a suspect reservoir to APT are reviewed. A new formula for evaluation of gas reservoir's potential for APT is presented; CAPT or coefficient of APT. Using information of some gas reservoirs, the comparison is made between predictions made by different formulas. Comparisons show that CAPT predicts true gas reservoir's potential for APT and is more accurate than other formulas.

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